Hydrocarbon wells, horizontal wells in particular, may have sections of wellscreens having a perforated inner tube with an overlying screen portion. The purpose of the screen is to block the flow of particulate matter into the interior of the production tubing. Despite the wellscreen, some contaminants and other particulate matter may continue to enter the production tubing. The particulate matter usually occurs naturally or is part of the drilling and production process. As the production fluids are recovered the particulate matter is also recovered at the surface. The particulate matter causes a number of problems in that the material may be abrasive or hazardous to the environment, thereby reducing the life of any associated production equipment and creating a disposal problem. By controlling and reducing the amount of particulate matter that is pumped to the surface, overall production costs are reduced.
Even though the particulate matter may be too large to enter the production tubing, the particulate matter may cause problems at the downhole wellscreens. As the well fluids are produced the larger particulate matter is trapped in the filter element of the wellscreens. Over the life of the well, as more and more particulate matter is trapped in the filter elements, the filter elements become clogged and restrict flow of the well fluids to the surface.
A method of reducing the inflow of particulate matter before it reaches the wellscreens is to pack gravel or sand in the annular area between the wellscreen and the wellbore. Packing gravel or sand in the annulus provides the producing formation with a stabilizing force to prevent any material around the annulus from collapsing creating additional particulate matter, it also provides a pre-filter to stop the flow of particulate matter before it reaches the wellscreen.
In a typical toe to heel gravel packing operation a screen and packer are run into the wellbore together. Once the screens and packer are properly located the packer is set so that it forms a seal between wellbore and the screen isolating the region above the packer from the region below the packer. The screen is also attached to the packer so that it hangs down in the wellbore forming an annular region around the exterior portion of the screen. At the bottom of the screen is a section of tubular that is blank but for the presence of gravel pack ports. The upper end of the screen is usually referred to as the heel and the lower end of the screen is usually referred to as the toe of the well.
Typically a washpipe subassembly is put together on the surface and then run into the wellbore where it stings through the packer and then run into the screen. The run in continues until the washpipe outlets are approximately aligned with the gravel pack ports in the blank section of tubular past the screens and near the toe of the well.
Once the washpipe is landed, a slurry, usually containing gravel, may be pumped down the well through the washpipe. When the gravel slurry reaches the outlets in the washpipe it exits the washpipe. The blank section of tubular may have an internal seal to help direct the gravel slurry through the gravel pack ports in the blank tubular and finally the gravel slurry flows into the packer and into the annular space created on the outside of the screen.
As the slurry travels from the toe of the well toward the heel along the outside of the screen, an alpha wave begins that deposits gravel from the toe towards the heel, all the while the transport fluid that carries the gravel drains to the inside of the screen. As the fluid drains into the interior of the screen it becomes increasingly difficult to pump the slurry down the wellbore. Once a certain portion of the screen is covered the gravel will start building back from the heel towards the toe, the beta wave, to completely pack off the screen.
After the annular area around the screen has been packed with gravel then the operator begin to reverse out. In some instances the operator may desire to backwash the interior of the screen to remove and excess gravel that may have been deposited in the interior of the screen assembly. In this case the ports in the washpipe that were depositing the sand slurry into the annulus are now raised above the internal seal and the operator pumps gravel free fluid down the annular area around the exterior of the washpipe to reverse the fluid inside of the washpipe back to surface thereby removing any the excess sand or gravel but leaving the gravel that was placed around the exterior of the screen in place.